1. Field of the Invention
The present invention relates to a device and a method used in a wireless communication system, and more particularly, to a device and a method of handling a hybrid automatic repeat request (HARQ) operation for an unlicensed band in a wireless communication system.
2. Description of the Prior Art
A long-term evolution (LTE) system provides high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, a radio access network known as an evolved universal terrestrial radio access network (E-UTRAN) includes at least one evolved Node-B (eNB) for communicating with at least one user equipment (UE), and for communicating with a core network including a mobility management entity (MME), a serving gateway, etc., for Non-Access Stratum (NAS) control.
A LTE-advanced (LTE-A) system is an evolution of the LTE system. The LTE-A system extends cell coverage of an eNB, increases peak data rate and throughput, and includes advanced techniques, such as carrier aggregation (CA), licensed-assisted access (LAA) using LTE, etc.
The eNB may communicate with the UE via an unlicensed band. However, resource in the unlicensed band is not always available due to that a channel is being used (i.e., channel busy). Operation rules designed for a licensed band may not be operated regularly in the unlicensed band. For example, a timing of a retransmission in the unlicensed band is not predetermined due to the channel busy. For another example, a UE may flush a hybrid automatic repeat request (HARQ) buffer for the retransmission, while the eNB does not know that the HARQ buffer of the UE is flushed. The eNB may request the UE to perform the retransmission associated with the flushed HARQ buffer. In this situation, the retransmission is failed since data in the flashed HARQ buffer is cleared.
Thus, how to handle a HARQ operation for (e.g., related to or in) the unlicensed band is an important problem to be solved.